This invention is directed to a method of stripping a resist mask (referred to hereinafter as ashing).
As is well known, a mask made of an organic photoresist, etc., is necessarily employed in a photo-lithography technique for fabricating semiconductor devices. After the photoresist is exposed to light for the photo-lithography technique the resist must be removed. A dry plasma etching method, particularly a downstream ashing method, is widely used for removing the used photo-resist. A downstream etching apparatus which can be employed is disclosed in U.S. Pat. No. 4,512,868 by the present inventor. This apparatus has been used for processing a silicon wafer, etc., where the wafer is protected from being bombarded by charged plasma particles.
The reaction gas used for ashing the organic photo-resist is, for example, an oxygen gas including a CF.sub.4 (carbon tetra-fluoride) gas. However, it is not always easy to remove the used resist. This is because the resist has been affected by its exposure to light during an exposure process and also its exposure to plasma during a prior silicon etching process, and therefore has become hardened. Furthermore, the CF.sub.4 gas is apt to undesirably etch the silicon wafer.
For improving the ashing rate, there is a method in which nitrogen gas is added to oxygen containing a reaction gas for increasing the number of oxygen atoms as a reaction species. In this method, for example, in which the reaction gas contains 10% nitrogen, the ashing rate is 0.3 .mu.m/minute at 180.degree. C. The increase of the ashing rate achieved with this method is still limited, however, because the reactive species are mainly only the oxygen atoms. The function of the added nitrogen gas is considered to increase only the number of oxygen atoms as the reactive species.
Another method for increasing the number of oxygen atoms is by employing a known plasma etching method in which a small amount of water vapor is added to the oxygen plasma gas. However, no extensive study or application of this method has been carried out.